Nitrogen gas has been increasingly used in many fields, including treatment of metals, production of semiconductors and a sealing gas in chemical industry. Pressure swing adsorption (referred to as “PSA”, hereinafter) is one of the most commonly used techniques in the production of nitrogen gas. In many cases, the technique is used to separate nitrogen from pressurized air and involves the use of porous carbon materials such as molecular sieve carbon. When the feed gas is air, two or more adsorption columns packed with molecular sieve carbon are generally used. The air is fed under pressure to one of the columns to cause oxygen to be adsorbed onto the molecular sieve, letting the unadsorbed nitrogen flow out of the column for collection. Meanwhile the pressure in the other column is reduced to cause the adsorbed oxygen to be desorbed. The adsorption and desorption of oxygen are alternated in the multiple columns. In this manner, nitrogen can be continuously collected by taking advantage of the difference in the adsorption rate between oxygen and nitrogen.
Referring to FIG. 1, a conventional PSA process is described wherein this process uses two adsorption columns to remove oxygen from air or other gas mixtures composed mainly of nitrogen, thus giving nitrogen as a gas product and regeneration of an adsorbent under normal pressure. In an adsorption step in a adsorption column 4, a feed gas such as air, is introduced via a feed gas inlet line 1 into a compressor 2 where it is compressed; and the compressed air is passed through a cooler 3 into the adsorption column 4. Each adsorption column is packed with molecular sieve carbon to serve as the adsorbent. In the adsorption column, oxygen in the feed gas is preferentially adsorbed onto the adsorbent and removed from the feed gas, allowing the remaining nitrogen to flow through to a product reservoir 6, from which the nitrogen is collected via a gas product line 16.
When one of the adsorption columns is operated in the adsorption step, the other is operated in the desorption step and is open to the atmosphere. Specifically, valves 7, 10 and 12 are opened and valves 8, 9, 11, 13 and 14 are closed when the adsorption column 4 is operated in the adsorption step. Part of the gas that flows out of the adsorption column 4 flows through an orifice 15 into the other column operating in the desorption step and purges the column.
After a predetermined period of time, the valves 7, 10 and 12 are closed. In the next pressure equalization step, the valves 11 and 14 are opened to release the residual pressure inside the adsorption column 4 into the adsorption column 5. Subsequently, the valves 8, 9 and 13 are opened and the valves 7, 10, 11, 12 and 14 are closed, so that the adsorption column 5 switches to adsorption step and the adsorption column 4 switches to desorption step. Once the adsorption in the adsorption column 5 comes to an end, the valves 8, 9 and 13 are closed and the valves 11 and 14 are opened to release the residual pressure inside the adsorption column 5 into the adsorption column 4, thereby equalizing the pressure in the system. This sequence operations are repeated cyclically to produce nitrogen product.
These valves are automatically opened or closed at the timings set by a timer, so that the nitrogen product is stored in the product reservoir 6 from which it is drawn out through the product gas outlet line 15 and is consumed. The gas (oxygen) adsorbed onto the molecular sieve carbon is desorbed as the pressure in the columns is reduced: once the valve 9 or 10 is opened, the adsorbed gas is released from the molecular sieve carbon and is discharged from the discharge line 17.
There are two ways to make PSA an industrially more advantageous process: to improve the separation performance of molecular sieve carbon or to improve the efficiency of PSA system. Of many PSA processes that have been proposed thus far few have taken into account both of these two approaches. In one process for making improved molecular sieve carbon, a hydrocarbon such as toluene is added to coke and the coke is processed at a high temperature. This causes carbon to be deposited within the pores of coke and as a result, the size of the pores is optimized. This improved molecular sieve is used to remove oxygen from the air to produce nitrogen gas (Patent Document 1).
Patent Document 1. Japanese Examined Patent Publication No. Sho 52-18675).
In another process, coconut shell carbon powder is granulated using a coal tar pitch binder and is carbonized. The granules are washed with hydrochloric acid, and impregnated with coal tar pitch. The carbonized carbon is then heated to make a molecular sieve carbon. This molecular sieve carbon is used to produce nitrogen gas from the air (Patent Document 2).
Patent Document 2: Japanese Examined Patent Publication No Sho 61-8004
In still another process, coconut shell carbon powder is similarly formed into granules using a coal tar pitch binder. The granules are washed with hydrochloric acid, and impregnated with creosote oil. The carbonized carbon is then heated to make a molecular sieve carbon. This molecular sieve carbon is used to produce nitrogen gas from the air (Patent Document 3).
Patent Document 3: Japanese Examined Patent Publication No. Hei 5-66886
On the other hand, some PSA processes focus on improving the efficiency of the process by modifying the design of the system. One such system uses a molecular sieve carbon with specifically defined performance and is designed such that the time at which the exhaustion valve is opened during the regeneration step under atmospheric pressure and the time at which the nitrogen gas product is passed through the adsorption columns are controlled to satisfy a predetermined relationship. The performance of the molecular sieve carbon is defined by determining the ability of the molecular sieve to adsorb oxygen or nitrogen. Specifically, this is done by leaving the molecular sieve in the presence of each gas alone under pressure for 1 minute and determining the ratio of the volumes of oxygen and nitrogen adsorbed by the molecular sieve (Patent Document No. 4).
Patent Document 4: Japanese Patent No. 2619839
Another such system uses a molecular sieve carbon with its performance defined by determining its ability to adsorb oxygen or nitrogen. Likewise, the performance is determined by leaving the molecular sieve in the presence of oxygen or nitrogen alone under pressure for 1 minute and determining the ratio of the volumes of oxygen and nitrogen adsorbed by the molecular sieve. In this system the product nitrogen gas amount taken out and the effective volume of the product storage tank are controlled so as to be correlated to the effective volume per one adsorbing tower, and the time required for the adsorption step is specified (Patent Document 5)
Patent Document 5 Japanese Patent No. 2623487
These improved PSA systems may be used in conjunction with any of the modified molecular sieve carbons to make the PSA process even more suitable for industrial applications. To this ends it is necessary to design small PSA systems that require minimum amounts of molecular sieve carbon and feed air.
Recently, a PSA system in which the pressure rise rate at the pressurizing step and the time at the pressure-equalizing step are controlled is developed. This facilitates effective generation of highly pure nitrogen gas (Patent Document 6). Another PSA system achieves improved separation performance by using a cylindrical molecular sieve carbon that is 0.5 to 1.5 mm in height (Patent Document No. 7).
Patent Document 6: Japanese Patent Application Laid-Open No. 2001-342013
Patent Document 7: Japanese Patent Application Laid-Open No. 2003-104720